Receiver functions in the western United States, with implications for upper mantle structure and dynamics

[1] Investigations into mechanisms driving surface tectonics commonly search for mantle sources, but few observations constrain flow in the upper mantle and transition zone. Here variations in the upper mantle discontinuities at 410 km and 660 km below the western United States are revealed through mapping depths of compressional-to-shear wave conversions recorded by broadband seismometers. The resulting image exhibits 20 and 30 km of topography on the 410and 660-km discontinuities, similar depth variations to those seen where subducting slabs of lithosphere reach the transition zone. The pattern of discontinuity topography imaged here does not correlate with the surface tectonics of the Rocky Mountains, Colorado Plateau, or Basin and Range Province, providing no support for upward and downward flow at transition zone depths controlling surface topography and deformation in this region, at least at scale lengths of a few hundred kilometers. Furthermore, we find no clear correlation between the depths of the 410and 660-km discontinuities. Undulations on the surfaces of both discontinuities appear to be spaced at distances of 800 km. If the topography were due only to lateral temperature differences, such differences would be comparable to those where slabs sink and might suggest separate convective flow above and below the transition zone. Alternatively, the topography may reflect lateral variations in composition. Variations in the sharpness of converted phases across the region offer some support for compositional heterogeneity, but the lack of a correlation between sharpness and depth casts doubt on this explanation for the variations in depth.